Need help with a sub project:

[EricH]

I've got two Dayton DVC 12's laying around, and I've decided to build a box for them. I was wondering if someone could tell me whether this will work out as I think it would.

I own an SVS PB12-Plus/2, and was going to copy the design. Same internal dimensions, bracing, damping, number of 4" ports and port length etc. I would use a 500w Bash amp. I could substitute the 4" ports for 3" as this set-up seems similar to SVS's PB12-ISD/2 which uses 3".

Is this a good idea? Does someone have a superior design, or can someone model this for me? I'm looking for a 20hz native tune.

Thanks, Eric.

[gatemplin]

Hi Eric,

Get me the T/S specs and I can run them through UniBOX for you.

Dont get your hopes up on using the exact same dimensions, and multiple 4" ports probably wont work (wont fit). It is probably best to tune the box for the best response and leave it there. Having the ability to have variable tuning points will take significantly more effort in the design stage and probably some prototyping aswell.

Keep in mind that bracing in commercial subs is a trade-off between reducing box resonance and colouration, and cost and complexity for mass-production. You can do better for not much more money, but weight and construction time increases.

[EricH]

Thanks a lot Graham. Here are the Dayton DVC12 specs:

Specifications: Power handling: 350 watts RMS/per coil, 600 watts total * Voice coil inductance: 1.81 mH * Nominal impedance: 8 ohm per coil / 4 ohm total * DC resistance: 2.69 ohms * Frequency range: 20-450 Hz * Magnet weight: 112 oz. * Fs: 21.7 Hz * SPL: 87.4 dB 1W/1m, 90.4 dB @ 2.83V/1m * Vas: 4.25 cu. ft. * Qms: 12.53 * Qes: .38 * Qts: .37 * Xmax: 15.1mm * BL 12.51 * Sd: .0464 m^2 *

I don't necessarily want variable tune, I just want it "flat" to 20hz or below. I will brace it very well, especially since I can't get my hands on any 1" mdf, only 3/4". Is doubling the 3/4" worth the strain on the back?

Slightly off topic, why does a variable tune sub like the SVS lose headroom when you plug a port? Is it because of air restriction due to less port area. Or because plugging a port effectively increases box size, while decreasing total port length, moving it away from the desired tune.

Thanks, Eric.

[gatemplin]

Originally Posted by EricH

I don't necessarily want variable tune, I just want it "flat" to 20hz or below. I will brace it very well, especially since I can't get my hands on any 1" mdf, only 3/4". Is doubling the 3/4" worth the strain on the back?

Flat to 20 Hz shouldn't be a problem. 3/4" is good, I used it on my sub, but it does flex, and so does 1.5". For this application, 3/4" should be kosher if the bracing keeps the unbraced panel widths as narrow as possible.

Slightly off topic, why does a variable tune sub like the SVS lose headroom when you plug a port? Is it because of air restriction due to less port area. Or because plugging a port effectively increases box size, while decreasing total port length, moving it away from the desired tune.

Thanks, Eric.

The sub losses headroom with the lower tuning points because the assistance from the vents is at a lower frequency. So at say 30 Hz, the output in the 16 Hz tune is just from the driver. Whereas in the 25 Hz tune, the vents are assisting the output at 30 Hz. Does that answer the question?

[gatemplin]

With three 3" flared ports, the tuning at 22 Hz, and the volume at 112L. The catch is that the ports need to be about 28 inches long. Now this is designed so that at 22 Hz at maximum power, there will be no port chuffing. Three 4" ports become obscenely long and two 4" ports are about 31" long. In this config, you get anechoic extension to 22-23 Hz, with room gain easily below that.

So, if the ports are too long for the box, one port has to be eliminated, or the box has to become larger than 112L.

I estimate that the SVS is about 6 cubic feet (170L). So if you built it to match, with the ports at the same lengths above, you will get kick-ass extension flat to 18 Hz anechoic, obviously even better in room.

I can post graphs later, but I have a physics exam tomorrow. That's enough procrastinating for me today. But I think you have a better idea of what you can expect now. You can always plug the ports for fun to get better extension, while sacrificing output.

[EricH]

That's very close to what I'm running on the SVS. Yes, it's about 6 cu.ft. and I'm running 20hz tune with one port plugged, so that is, in effect, two 4" ports.

So should I tune to 22 or 20hz? Also, for future reference, is it advisable to tune to the Fs of the driver?


"The sub losses headroom with the lower tuning points because the assistance from the vents is at a lower frequency."


Yes, that makes sense. Thanks for the "tutoring", and good luck on the exam today. :-)

[gatemplin]

Well, I told you initially not to copy the SVS ... but it turns out that these model very similar (less output). So, I suggest you copy the SVS exactly. Same ports, same length. The bracing is should be copied too, but you can add a little more if you like. This sub will do well with tuning from 25-16 Hz, so that fits too.

So should I tune to 22 or 20hz? Also, for future reference, is it advisable to tune to the Fs of the driver?

In general, you should always model first. It also depends on Qts, which is how much the driver tends to resonate at Fs. And it depends on the application too. Sometimes it does turn out to be exactly Fs, but I have designed speakers with Fc more than 10 Hz below Fs.

Edit: Oh, and thanks for the best wishes on the exam. It's physics which is my forte, so I'll do fine. But I have Linear Algebra and Chemistry coming up, and I'll need all the luck I can get on those.